Coaxial transmission lines and coaxial resonators are used in many types of microwave and radio-frequency (“RF”) filters, including both bandpass and bandstop implementations. Examples of prior-art tunable filters (herein also referred to as “factory adjustable filters”) are documented in Snyder, R. V., “A Compact, High Power Notch Filter with Adjustable F0 and Bandwidth,” IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, Vol. 42, No. 7, July 1994 and Snyder, R. V., “Quasi-Elliptic Compact High-Power Notch Filters Using a Mixed Lumped and Distributed Circuit,” IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, Vol. 47, No. 4, April 1999. These articles are incorporated herein by reference in their entirety.
FIG. 1 illustrates a prior-art factory adjustable notch filter 100 that utilizes prior-art factory adjustable coaxial resonators. Filter 100 comprises a plurality of coaxial resonators 120, 140, and 160, each of which are capacitively coupled to conductive loops 136 via respective plates 136A, 136B, and 136C. The capacitive couplings are illustrated in FIG. 1 as respective open circuits 132A, 132B, and 132C. Loops 136, which may be sections of coaxial cable, are capacitively coupled to ground by plates 134A, 134B, and 134C. Thus, plates 134A and 136A form a capacitor 135A; plates 134B and 136B form a capacitor 135B; and plates 134C and 136C form a capacitor 135C. Coaxial resonators 120, 140, and 160 are contained with a housing 138.
A description of the construction of coaxial resonator 120 will now be provided. It is understood that coaxial resonators 140 and 160 are similarly constructed. Coaxial resonator 120 comprises an outer conductor 122, an inner conductor 124, an insulating layer 126, a short circuiting mechanism 128 near end 130, and an open circuit 132A (described above) opposite end 130. Short circuiting mechanism 128 is secured to inner conductor 124 and slidably connects inner conductor 124 to outer conductor 122, thereby providing a short between outer conductor 122 and inner conductor 124. Extension 130A is disposed about inner conductor 124 between shorting mechanism 128 and end 130. Short circuit 128, insulating layer 126, open circuit 132A, and loading capacitor 135A connected between open circuit 132A and ground (not shown) determine the electrical length of resonator 120.
The dielectric properties of insulating layer 126 are important in the electrical length of resonator 120. In one prior-art embodiment (now described), insulating layer 126 is formed from a soft dielectric such as polytetrafluoroethylene (herein “PTFE” or “Teflon®”). In such an embodiment, the maximum dielectric constant of insulating layer 126 achievable is about 2.2, but unavoidable air gaps between conductors 122 and 124 and insulating layer 126 reduce this value to perhaps 2.0.
With respect to coaxial resonator 120, because insulating layer 126 is formed from PTFE which is lubricious, the assembly of inner conductor 124, short circuiting mechanism 128, and insulating layer 126 may be easily adjusted (slid in or out of outer conductor 122) to alter the effective electrical length of resonator 120. Extension 130A acts as a handle and aids in moving this assembly. Once adjusted, inner conductor 124 is secured by tightening set screw 139 to prevent further movement. Similar adjustments are made to coaxial resonators 140 and 160 to tune or adjust resonator 100.
As the ambient temperature of coaxial resonator 120 changes, the effective dielectric constant of insulating layer 126 also changes. This change in dielectric constant is due to the high thermal coefficient of expansion (“TCE”) for PTFE, which TCE exceeds 100 parts per million (“PPM”) per degree Centigrade. As the ambient temperature decreases, the PTFE in insulating layer 126 shrinks at a much great rate than conductors 122 and 124 (typical conductor TCE=20 PPM), thereby introducing air gaps (not shown) between insulating layer 126 and conductors 122 and 124. Because the dielectric constant of air is less than that of PTFE, the introduction of air gaps between insulating layer 126 and conductors 122 and 124 effectively reduces the dielectric constant of insulating layer 126. Conversely, as the ambient temperature increases, the higher rate of expansion for PTFE causes compression of the PTFE in insulating layer 126 between conductors 122 and 124. Because PTFE is a highly thermoplastic (and thus compressible) material, the effective dielectric constant of insulating layer 126 increases.
FIG. 2 illustrates the frequency response of a conventional dual notch filter that uses the coaxial resonators described above with respect to FIG. 1. As can be seen in FIG. 2, as the temperature of the filter changes, the frequency response changes. For example, the attenuation of a 1008 MHz signal is −4.716 dB when the filter is at −40 C. When the temperature is raised to 55 C, the attenuation becomes −3.373 dB. The change in frequency response resulting from a change in temperature illustrates that the effective dielectric constants of the insulating layers of the resonators—and therefore the effective electrical lengths of the resonators—changes as temperature changes. Because of the effect of temperature on the frequency response, such filters must be designed with a “guardband,” so that either rejection or insertion loss is maintained as temperature changes.
Coaxial resonators have applications in modern military hardware. The nominal electrical length of resonator 120 is determined by the maximum value of the dielectric constant of insulating layer 126. As described above, for PTFE and similar soft, i.e. plastic, dielectrics, that value is about 2.2. Thus, a resonator designed for an electrical length of 80 degrees at 1030 MHz would have a physical length of about 1.76 inches. Although the resonator need not be straight, a physical length of 1.76 inches per resonator is required to provide such an electrical length. The temperature variation of such an element is perhaps +/−1.5 MHz as temperature varies from −55 to +85 C, a typical military range requirement. The guardband (described above) accommodates this effect on the frequency response.